My grandfather had recently died, and I was spending time at my grandmother’s house. After 60 years of marriage, she wasn’t used to being alone or to the sadness an empty home can bring.

Determined to help her in any way I could, I moved into her spare bedroom for a few days. As night came, I tucked her into bed and turned out the light - a task she had done for me on countless occasions growing up. The role reversal saddened me but also gave me an overwhelming urge to protect one of the most important women in my life. I lay down in the next bedroom and listened to her muffled sobs.

I woke up a few hours later, feeling cold. As I went to pull the blankets up around me, I realized I couldn’t move. I began to panic. What was happening to me? Why was my body paralyzed? I tried to lift my arms: Nothing. My head was cemented to the pillow, my body embedded, frozen.

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Then the pressure came, pushing against my chest. The more I panicked, the harder it became to breathe. Like something out of a bad horror movie, I tried to scream, but no words came out. Unable to move my eyes, I had no option but to stare upward into the darkness. I couldn’t see anyone else, but for some reason it felt as if I had company. There was a hidden presence and it was tormenting me, refusing to let me go. After what felt like hours but was probably just a few minutes, I was able to move again. Shaking, I switched the bedroom light on and sat upright in bed until morning came.

“Okay, imagine this,” I said to a friend the following evening. “I went to sleep as normal but woke up with Lord Voldemort squatting on my chest.” She laughed. “Umm, Jen, are you sure this wasn’t just a nightmare?” I considered her question. I’d had nightmares before, but this? This was different. This actually happened while I was awake.

“I don’t think so,” I replied. Deep down I was certain that I had been conscious, fully present, just immobilized for what felt like an eternity.

Others I spoke to suggested that perhaps grief was playing a factor. “You’re going through a stressful time, darling,” my mum said, doing her best, as always, to reassure me.

I didn’t want to arrange a doctor’s appointment - it seemed unnecessary to take up valuable National Health Service time (I live in London) - so I turned to Google. (Continued below.)

Soon enough, I was reading stories from people who knew exactly what I was talking about. “Look at this,” I said to my mother. “Loads of people online have had this, too.” Relieved, I realized we weren’t all unstable or dreaming or joking: We were suffering from sleep paralysis.

Sleep paralysis occurs when the mind wakes up but the body remains asleep. This causes temporary immobility and, in many cases, intense hallucinations. For some people the paralysis lasts seconds, for others several minutes.

Adrian Williams, a professor of sleep medicine at King’s College London and a member of the medical team at the London Sleep Center, says sleep paralysis is a “normal phenomenon that is not dangerous but is distressing.”

As we sleep, our bodies alternate between REM (rapid eye movement) sleep and NREM (non-rapid eye movement) sleep. During the REM stage, our brains are highly active; as a result, this is when our most elaborate dreams occur.

“During dreaming sleep, the body is paralyzed to prevent us from acting out our dreams. Occasionally the body gets confused and the brain wakes yet the paralysis persists,” Williams said.

Episodes are almost always accompanied by a feeling of intense pressure on the chest. Naturally, the inability to breathe rouses feelings of panic and despair. “Because of the paralysis, the only breathing muscle that is working is the diaphragm. There’s often a sense of inadequate breathing because the chest muscles are not working,” Williams said.

Around half of the population has experienced sleep paralysis, he said. Some will notice it frequently, others just once or twice.

Relatively few seek treatment. The London Sleep Center sees only about one sleep paralysis patient every month. “By the time a person goes to see a doctor, the paralysis is usually happening often,” Williams said. “Often patients do not know they’re suffering with sleep paralysis, which is why they come.”

Sleep paralysis is commonly linked with narcolepsy, a rare condition that affects the brain’s ability to regulate the sleep-wake cycle. Williams estimates that two-thirds of narcoleptics also have sleep paralysis.

According to information from the National Health Service, sleep paralysis can be triggered by anxiety, stress and depression - which may explain why my first encounter with the condition came during a time of grief. Those with irregular sleeping patterns are more at risk than others of experiencing the disorder while falling into or waking up from sleep.

“There is no antibiotic to make sleep paralysis go away,” Williams said. “The first thing to do is to focus on making sleep better in a behavioral way.”

Williams said that the condition can be passed through generations but is not gender-related. “I’ve seen three or four families with this problem,” he said.

Many cultures have blamed sleep paralysis on the underworld and mysterious creatures such as the “old hag” and the “devil in the room.” Henry Fuseli’s 1781 painting “The Nightmare” is frequently associated with the disorder: He depicts a sleeping woman sprawled helplessly on a bed as an ogre sits on her chest. (Continued below.)

In Thailand, some believe that breathing difficulties at night are caused by the Phi Am spirit - a ghost that sits on your chest and crushes you. Guam has legends of the Taotaomona, a forest vampire spirit that seeks to protect Earth. Those who disrespect the island, the stories say, will be strangled in their sleep.

Williams, who said sleep paralysis occurs more commonly when sleeping on the back, suggests that the problem may by avoided by sleeping on the side. Episodes can be interrupted by touch, he said, so a bed partner may be able to intervene.

When sleep paralysis is happening frequently and efforts to correct it have not been successful, he said, antidepressants are sometimes prescribed, not necessarily because the patient is depressed but in an attempt to suppress REM sleep.

Some people have told me that they are sometimes able to blink themselves out of their paralysis. Others find that slowly moving their fingers and toes helps break the spell.

Since that night in 2012, I have experienced sleep paralysis at least 10 times. Once it struck three times in a single night. It’s terrifying and it’s exhausting.

Now I avoid caffeine before bed and listen to relaxation music while falling asleep. I also keep a sleep journal in which I write down descriptions of episodes. My most recent encounter came over Christmas break. An excerpt:

“3:30 a.m.

“I’m in a room with a man I don’t know. He asks if I want a drink and leaves to make me one when I say yes. But as he leaves, he turns the lights out, plunging me into darkness.

“Instantly I’m uncomfortable, it’s time to go home. . . . but I’m stopped by the other people who are now somehow in the room with me. They emerge from the shadows and begin grabbing me. They’re pulling my fingers, snapping my wrists, they’re tugging my hair until it falls out in patches onto the carpet below. I’m frozen, unable to escape. A prisoner.

“My attackers disappear and the lights are back on, but I’m stuck to the floor of this unfamiliar room. I’m lying on my back unable to scream. I’m trying to shout but nothing comes out. I begin smacking my bare arm against a wooden desk, desperate to draw attention to myself.

“Bang bang bang. My arm turns purple, it bleeds. I smash it against the desk until there’s a hole in my skin and I can see the bone underneath. Nobody hears me and nobody comes.”

It’s over just as quickly as it started. I’m awake and I’m safe at home. My fingers aren’t broken and there is no one else in the room. I check my arm. It’s cold but not bloody or bruised. Sleep paralysis strikes again.